Lanefan
Victoria Rules
Part of the DM's job.Yeah, because DM's love splitting the party so much... I mean, who doesn't want to run two *separate* campaigns just to coddle a drama queen player?
That said, the Harry "campaign" probably doesn't have to amount to much - just a couple of dice rolls now and then to see how he's doing and-or whether he's doing anything that might interact with the main party in any way. Unless Harry makes it his goal to take out the rest of the party, in which case the DM will have to run Harry's campaign in more detail and then also DM Harry's eventual attack (with Harry still being played by his player) when it comes.
Harry operating alone also gives the DM some story options - he could, for example, end up as some sort of operative gathering information which eventually gets indirectly passed on to the party. Or he could intentionally or otherwise become a distraction, allowing the party freedom to act elsewhere. Etc.
And, "coddle a drama queen player" is more than a little harsh, given that we don't know any history of their group and-or whether this is a regular thing for this player or just a one-off.
Demand? No. But that doesn't and shouldn't mean Harry must become an NPC.Sorry, when you aren't fit to be in the party, you don't get to demand your own private campaign.
Lanefan